1. Field of the Invention
The present invention relates to the problem of slow initializations when time is uncertain, and more particularly to using a GPS-calibrated real-time clock left running during power-down cycles to supply an accurate time estimate for both halves of a hybrid GPS-receiver mobile-phone during initialization.
2. Description of the Prior Art
Most consumer electronic devices are mass produced and their sales are very dependent on how they are priced. One way to offer better prices is to reduce manufacturing costs, e.g., the cost of labor and the components used. Combinations of navigation satellite receivers and communication devices are now available, and many conventional commercial products provide one set of crystals and crystal oscillators for the navigation satellite receiver part, and a separate set for the communication devices.
Global positioning system (GPS) receivers use signals received from several earth-orbiting satellites in a constellation to determine user position and velocity, and other navigational data. A prior art navigation receiver that has just been turned on does not yet know where it is, how much its crystal oscillator is in error, nor what time it is. All these are needed to find and lock onto the satellite transmissions, and so a search must be made of the possibilities. The most probable are searched first to save time.
High-sensitivity GPS receivers are a problem when the initial time or frequency uncertainty is large. Finding signal energy when the signal energy is extremely faint requires making smaller steps and dwelling at each step longer. So having a better initial estimate of the local reference oscillator can improve time-to-first-fix.
GPS receivers with signal levels better that −145 dbm can readily lock onto a strong GPS satellite vehicle (SV) to decode the NAV-data. Such yields the SV ephemeris and position. After than, the total pseudorange needs to be formed from the hardware codephase. Conventional GPS receivers determine the integer millisecond and so-called z-count.
When signal levels are roughly no better than −145 dbm to −150 dbm, a practical high-sensitivity GPS receiver can employ pattern-matching tricks to get a z-count or integer millisecond for an anywhere-fix.
GPS receivers that are locked onto and tracking one or more GPS satellite vehicles (SV's), know time to very high accuracy. This is because the GPS system is based on atomic clocks that set the time and frequency references used. The coarse acquisition (C/A) transmitted by the SV's repeats every one millisecond of propagation wavelength, and so there is a basic ambiguity as to which millisecond a GPS receiver is observing. If the integer millisecond is known, e.g., time is known to better than one millisecond, then the integer ambiguity does not need to be solved. The z-count is known. Skipping the steps to find z-count and set the integer millisecond can save a tremendous amount of time and effort in a GPS receiver working on providing its first navigation solution fix after a cold start.
There is a need for a less expensive-to-produce combination of a navigation satellite receiver and a communication device.